Temper embrittlement in HY-80 steel: Microstructure, magnetic and microhardness properties

Abstract

Temper embrittlement is characterized by the migration of impurity elements and carbides towards the grain boundaries of alloy steels, such as HY-80 steel. The accumulation of inclusions at grain boundaries causes dramatic reductions in impact toughness due to the increased susceptibility of intergranular fracture. This kind of failure can be catastrophic for submarines under high pressure with hulls constructed of HY-80 steel. The detection of temper embrittlement is crucial for the safety of the crew. Currently, the most common method of detecting temper embrittlement is destructive, which is costly and inefficient. Non-destructive methods of temper embrittlement detection are more desirable, however, there are no widespread methods currently in use. One possible non-destructive method utilizes Magnetic Barkhausen Noise (MBN). This method was investigated by testing eight HY-80 samples for MBN energy, microhardness, and microstructural changes before and after a heat treatment at 525 °C for a variety of holding times. Scanning electron microscopy showed carbide migration to the grain boundary and spheroidization with increased holding time. Microstructural quantification was performed using computer vision to extract useful data on precipitate behavior during heat treatment. The microhardness of each sample was measured using Vickers microhardness testing before and after heat treatment to further characterize the level of temper embrittlement. Microhardness changes with holding time were attributed to the change in precipitate density and the migration of the precipitates to the grain boundary during temper embrittlement.